Carbon nanotube apparatus and method of carbon nanotube modification

ABSTRACT

Carbon nanotube apparatus, and methods of carbon nanotube modification, include carbon nanotubes having locally modified properties with the positioning of the modifications being controlled. More specifically, the positioning of nanotubes on a substrate with a deposited substance, and partially vaporizing part of the deposited substance etches the nanotubes. The modifications of the carbon nanotubes determine the electrical properties of the apparatus and applications such as a transistor or Shockley diode. Other applications of the above mentioned apparatus include a nanolaboratory that assists in study of merged quantum states between nanosystems and a macroscopic host system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to, and claims the benefit of priority to,U.S. Provisional Application No. 60/632,394 filed Nov. 29, 2004,entitled Carbon Nanotube Apparatus And Method of Carbon NanotubeModification, by Francisco Santiago, Victor Gehman, Jr., Karen Long, andKevin Boulais, the contents of which are incorporated herein byreference.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein was made in the performance of officialduties by employees of the Department of the Navy and, thus, theinvention disclosed herein may be manufactured, used, licensed by or forthe Government for governmental purposes without the payment of anyroyalties thereon.

BACKGROUND

1. Field of the Invention

The following description relates, in general, to an apparatus includinga carbon nanotube, and a method of modification of a carbon nanotubeand, more particularly, to a modification of carbon nanotubes tofacilitate use in electrical applications.

2. Description of the Related Art

The structural arrangement of elements, such as carbon, can facilitatethe use of the element in varied applications. In nature the elementcarbon exists in several different configurations, e.g.,polycrystalline, graphite, and diamond. In the last two decades, newforms of elemental carbon have been synthesized. One example is thefullerene structure having an arrangement of carbon atoms, i.e., sixtycarbon atoms in the shape of soccer balls with diameters of less thanten nanometers.

Another example of a synthesized form of carbon are carbon nanotubes.Material structures such as carbon nanotubes have attracted significantattention in the scientific community due to their unique properties andpotential applications. A carbon nanotube can be visualized as a singlesheet of graphite atoms that have been rolled to form a cylinder. Theelectrical properties of the nanotube are dependent in part on arelationship of the ends of the sheet after the rolled sheet is formed.

Chirality is a property of molecular systems in which a mirror image ofa system is not symmetrical with itself, e.g., a common hardware screw.In formation of a carbon nanotube, a chiral tube can result when ends ofthe sheet are displaced. A chiral vector can be used to represent anamount of carbon lattice displacements during a rolling-up of the sheet.The chiral vector represents a dimensional lattice that determines someproperties of the carbon nanotube.

While, carbon has been attached to a silicon substrate with gold orother metals, and ohmic contacts have been used between a carbonnanotube and a host material, however, they have not proven effective ina merging of a quantum electronic states of the carbon nanotube with thehost material.

To facilitate use of carbon nanotubes, a nanolaboratory and a method tomodify nanotubes is desired.

SUMMARY

In one general aspect, techniques for providing a nanolaboratory and amethod of modifying a carbon nanotube are described.

One method of connecting a carbon nanotube to a substrate includesexposing the carbon nanotube to a vapor from a substance deposited onthe substrate, wherein a first radical of the substance is selected froma family of a periodic table having a spherically-shaped probabilitydistribution valence orbital.

In another aspect, a nanolaboratory includes a substrate, a substanceattached to the substrate, and a carbon nanotube positioned on thesubstance and at least partially merged with the substrate. Thenanolaboratory can be used to investigate nano-macrosystems integration.

In yet another aspect, a method of modifying a carbon nanotube tosubstantially permit the nanotube to have graphite-like electricalproperties is provided. The method includes exposing the nanotube to avapor from a substance to selectively remove a carbon atom.

In another general aspect, a graphite-like carbon nanotube is provided.The graphite-like carbon nanotube includes a carbon nanotube having acarbon atom selectively removed. Carbon nanotube systems with multipleelectronic profiles are provided.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing a method for modifying a carbon nanotube;and

FIG. 2 is a block diagram showing a nanolaboratory.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a flowchart showing a method 100 of modifying a carbonnanotube. The method 100 includes depositing a substance on a substrate(operation 110), positioning carbon nanotubes on the substance(operation 120), and raising the temperature of the substance so that anelement of the substance is at least partially vaporized (operation130).

In one implementation, the substrate is Si and the substance is BaF₂film deposited in a shape of triangular islands. The carbon nanotubesare positioned on top of the islands of barium fluoride. When thetemperature is raised, the fluoride evaporates from the barium fluoride,and the fluorine etches the carbon nanotube. The barium is used to linkthe carbon nanotubes and Si. Thus, the BaF₂ is used in protection of theBa—Si layer, in defining the bonding site, and in supply of the fluorineto etch the carbon nanotubes. Alternatively, other combinations ofradicals selected from the family II and family VII from the periodictable can be used, e.g, CaF₂, SiF₂.

A sector of the carbon nanotube that is in contact with a border of theisland is connected to the silicon with a bond resulting in asilicon-barium-carbon layering. While a rolling along a diagonal resultsin a net chirality, i.e., a screw structure and changes property of ananotube to a semiconductor, if a carbon nanotube is rolled in a mannerso as to stretch the bonds, the electrical properties are changed in amanner so the nanotube does not resemble graphite. In the fusing of thecarbon nanotube to the silicon, according to one implementation, thecarbon nanotube performs electrically more like graphite.

An issue in the field of nanotechnology is the prediction of emergingphenomena due to the interaction of a number of individual nanosystems.A nanolaboratory may assist in a study of merged quantum states betweennanosystems and a macroscopic host system.

FIG. 2 is block diagram showing a nanolaboratory 200. The nanolaboratoryincludes a substrate 210 having a substance 220 deposited on thesubstrate 210 and carbon nanotubes 230 positioned on the substance 220.

In one implementation, a merging of quantum states of carbon nanotubeswith a semiconductor crystal bonds the carbon nanotube with an elementthat features a spherically shaped probability distributions valenceorbital, for example, elements found in families I and II of theperiodic table. According to one aspect, family II elements, e.g., Mg,Ca, Sr, and Ba are used because of their having two electrons in theirshell. In reacting a family II element like Ba to an end of a carbonnanotube, one of the electrons of the Ba will bond with the carbon andthe other electron of the Ba will bond with a substrate or material tobe attached. Carbon nanotubes can have electrical properties that aremetallic or semiconducting depending on the chiral properties anddiameter. Applications of the carbon nanotube include acting as acarbon-nanotube transistor or Shockley diode.

In one implementation, a carbon nanotube is modified to facilitate theattachment of the carbon nanotube to a surface with chemical bonding.Such a modification permits the merging of carbon nanotube quantumstates with a substrate, e.g., a semiconductor crystal. This mergingproduces a nanolaboratory that, for example, can be used in study ofinteractions between large number of nanosystems.

Merging of the quantum states of large number of carbon nanotubes with asemiconductor crystal can be done by a chemical link using molecularbonds. However, direct bonding between carbon nanotubes andsemiconductor crystals is not practical due to the nature of thesymmetry of the valence bonds of both systems since in both cases thevalence states have probability distributions along specific directionsnot compatible with each other. According to one implementation, bariumis attached to a carbon nanotube. In addition, barium can be attached toother semiconductor surfaces like silicon (Si) and gallium arsenide(GaAs).

In one implementation, carbon nanotubes are modified so as to form acarbon-nanotube transistor. The method comprises connecting carbonnanotubes directly to silicon using Ba as a chemical link.

According to one implementation, production consistency is improvedregardless of the carbon nanotube. A degree of modification of chiralitychanges the electrical properties of the carbon nanotube so as to form,for example, a semiconductor or if a single sector is modified a diode.Further, only the electrical properties of the tube are effected, thatis other properties such as optical properties are not effected.

In one implementation, the removal of carbon atoms is selective. If sucha selective removal is applied only in a sector of the carbon nanotubean effective resistor is formed. According to another aspect, a diode isformed. For example, a silicon wafer is patterned with barium fluoridefilm by photolithography. Carbon nanotubes are positioned in thesolution on the wafer, and the wafer is spun. The wafer is placed in avacuum furnace and heated, and the fluoride evaporates and reacts withthe carbon nanotubes.

In one implementation, a carbon nanotube is positioned with bariumfluoride on one side forming a diode. For example, a carbon nanotube ispositioned in a very narrow sector of the barium fluoride and flashed.Thus, varied implementations are possible dependent on the patterning ofthe barium fluoride.

In another implementation, carbon nanotubes are connected to silicon(Si) crystals using the element barium as an atomic link between thecarbon nanotube and Si. The attachment of carbon nanotubes to Si is doneby exposing the carbon nanotube to a hot vapor of barium fluoride (BaF₂)from a film of the fluoride salt that has been previously deposited on aSi substrate that holds both the film and the nanotubes under ultra-highvacuum conditions. The substrate, film and nanotubes are heated inside avacuum chamber with a pressure below atmospheric pressure.

When a deposited film is used, there is a relation between the amount oftime of the reaction and the temperature used for the reaction withhigher temperatures requiring a lesser amount of time. An advantage ofbeing able to react the carbon nanotubes with deposited films of bariumfluoride films is the ability to control the placement of a potentialcarbon nanosemiconductor link.

In one implementation, a method to connect carbon nanotubes to galliumarsenide (GaAs) crystals comprises using the element barium as an atomiclink between the carbon nanotube and GaAs. The method facilitates theattachment of carbon nanotubes to a GaAs crystal surface via chemicalbonding and allows the merging of carbon nanotube quantum states withthe GaAs crystal to produce a nanolaboratory to help the study ofinteractions between large number of nanosystems. The attachment ofcarbon nanotubes to GaAs includes exposing the carbon nanotube to a hotvapor of barium fluoride (BaF₂) from a film of the fluoride salt thathas been previously deposited on a GaAs substrate that holds both thefilm and the nanotubes under ultra-high vacuum conditions.

In another implementation, a method for the molecular modification ofcarbon nanotubes with the element barium for the attachment of carbonnanotubes to electronic materials like semiconductors, insulators, andconductors using direct chemical bonding is provided. The attachment ofbarium to carbon nanotubes includes exposing the carbon nanotube to ahot vapor of a barium salt under ultra-high vacuum conditions. Thesource of the fluoride vapor is the sublimation of the solid form of thecompound that can come from a heated container or from a film of thefluoride salt that has been previously deposited on a substrate thatholds both the film and the nanotubes.

An aspect of being able to react the carbon nanotubes with depositedfilms of barium fluoride films is the ability to control the placementof a potential carbon nanotube semiconductor link. In oneimplementation, properties of metallic carbon nanotubes are locallymodified so they will be similar to the electronic properties ofgraphite while the rest of the tube retains its original properties.Such a method can be used in manufacture of, for example, a nanocarbonresistor.

By reacting the carbon nanotubes with deposited films of barium fluorideon a substrate film, the placement of the modified carbon nanotube iscontrolled. According to an aspect of the present invention, electronicproperties of semiconducting carbon nanotubes are locally modified sothey will be similar to the electronic properties of graphite, while therest of the tube retains its original properties. In such a method, ananoShockley diode, for example, can be produced.

A structure of a one dimensional lattice defined by the chiral vectordetermines if the carbon nanotube will conduct electricity like a metalor like a semiconductor. If, the periodicity of the chiral lattice isbroken, but the structural tube integrity is conserved then a graphitelike nanotube will be formed. In one implementation, a chiral latticecan be broken by carefully reacting the carbon nanotube with a vaporthat removes selective carbon atoms and replace them with a metallicatom. When carbon nanotubes are exposed to BaF₂ the fluorine atoms reactwith carbon and barium is left attached with the neighbor carbon atomsvia a carbide like bond breaking the chiral lattice and the onlyperiodic structure left is the graphite lattice. As result the tubeconducts electricity like graphite.

If a carbon nanotube that conducts electricity like a semiconductorexperiences a local modification to a graphitic region, a phenomenonoccurs in the interface between the graphitic sector and thesemiconducting sector that creates an energy barrier. This sector isknown as a Shockley contact. The junction in thegraphitic-semiconducting interface will create a nanodiode.

In one implementation, the selective removal of carbon atoms from acarbon nanotube is done by exposing the carbon nanotube to a vapor froma substance such as a family VII substance, e.g., a fluoride saltincluding family II atoms, e.g., barium fluoride (BaF₂), strontiumfluoride (SrF₂), and calcium fluoride (CaF₂); a chloride salt such asbarium chloride (BaCl₂), strontium fluoride (SrCl₂), and calciumchloride (CaCl₂); or an iodide salt such as barium iodide (BaI₂),strontium iodide (SrI₂), and calcium iodide (CaI₂)

In one implementation, only a sector of the carbon nanotube covers thefilm with the sector of the nanotube covering the film to be the sectorthat will become like graphite once the reaction takes place. Thesubstrate, film and nanotubes are heated, for example, to a temperatureabove a sublimation temperature. There is a relation between the amountof time of the reaction and the temperature and pressure used for thereaction with higher temperatures causing sublimation in a lesser time.In addition, the sublimation can be inside a vacuum chamber. Thus, byreacting a carbon nanotube with a film of barium fluoride that isdeposited on a substrate the placement of the of the modified carbonnanotube is controlled. The vapor comes from a film of the fluoride saltthat has been previously deposited on a substrate that holds both thefilm and the nanotubes. However, it is important that only a section ofthe carbon nanotube cover the film feature. A sector of the nanotubethat covers a sector of the film will develop graphite-like electricalproperties once the reaction takes place.

In one implementation, surfaces of silicon wafers are chemicallymodified to allow the epitaxial growth of BaF₂ using molecular beamepitaxy (MBE). A sample containing 2D single crystal islands of BaF₂ iscovered, for example, with carbon nanotubes with an average coverage of10 nanotubes per cm². The samples are transferred to an outgasingstation inside the MBE system and heated.

The chemical interactions between carbon nanotubes and BaF₂ vapors canbe studied, for example, using x-ray photoelectron spectroscopy (XPS)and atomic force microscopy (AFM). XPS shows that before heating only asmall amount of contaminants are present on the nanotubes, and changesof the carbon nanotubes occur after heating in the electronic spectrum.

BaF₂ dissociates and the fluorine attacks the carbon nanotube surfacetaking away some of the carbon and leaving Ba in the affected areas.Such reaction changes the curvature of the nanotubes due to differencesbetween the C—Ba bond and C—C bond creating sectors of flat graphiticbonds. In one implementation, carbon nanotubes are exposed to hot BaF₂vapor from films deposited on silicon. The films may be epitaxial filmsthat are growing a crystal layer of one mineral on the crystal base ofanother mineral in such a manner that its' crystalline orientation isthe same as that of the substrate. Carbon nanotubes, when exposed to hotBaF₂ vapor, will react forming a Ba—C bond plus a modification of theelectronic structure of the carbon nanotubes toward graphite.

In one implementation, a method is provided to connect carbon nanotubeswith known semiconductor surfaces using chemical bonds. This methodpermits study of optical phenomena. Epitaxial films of barium fluoride(BaF₂) on both silicon and GaAs can be grown. During the process thereis an intermediate layer of elemental barium bonded to the surface thathelps reduce the lattice mismatch between BaF₂ and the semiconductor.

In one implementation, a bond is created between carbon, barium andsilicon via thermal activation by exposing carbon nanotubes to hot BaF₂vapor under ultrahigh vacuum from epitaxial film islands deposited onsilicon. In one implementation, nanotubes are dispersed in a surfactant.For example, the nanotubes can be dispersed in a surfactant of Triton Xin a concentration of 5.79 mg/mL. The Triton X can be removed from thecarbon nanotubes by solvents including water, ethyl isopropyl alcohol,and toluene in ultrasonic bath. However, such solvents can leave aresidue of the surfactant on the surface of the carbon nanotubes asverified by atomic force microscopy (AFM) and x-ray photoelectronspectroscopy (XPS). Other methods of surfactant removal includeultrasonic cleaning with methanol.

In one implementation, removal of the surfactant is by a dispersing,centrifuging, and decanting of the carbon nanotubes. For example, asolution of HCl 5.0N standardized solution, 5 mL of carbon nanotubedispersion to can be mixed to 10 mL of HCl. The solution can beagitated, e.g., for 15 seconds, placed in a centrifuge and spun, forexample, at an effective centrifugal acceleration of 21,000× g for 15minutes. The carbon nanotubes can be separated by a decanting thesolution on top, and the process is repeated a number of times, e.g.,seven, with distilled water instead of HCl. Further, the carbonnanotubes are dispersed in methanol to remove the residue of Triton X.

The carbon nanotubes dispersed in methanol are deposited on a wafer,e.g., (001) silicon wafer covered with BaF₂ islands. Alternatively, onlya portion of the wafer can be covered with carbon nanotubes with anaverage density of 10 carbon nanotubes per square micron with anotherhalf used as experimental control to monitor the chemical changes ofbarium during thermal activation.

The BaF₂ islands are grown using molecular beam epitaxy (MBE). Forexample, to grow the islands, the wafer is placed inside a chamber witha residual pressure less than 4.0×10⁻¹² mbar and heated, e.g., to 750°C. and exposed to BaF₂ vapor using a vapor flux of 10¹⁰ molecules percm² per second. An environment is created whereby BaF₂ vapor reacts withsilicon and creates a two dimensional Ba—Si layer that serves as a hostto epitaxial BaF₂ films. The BaF₂—Si reaction is self-limiting and onceall available surface sites are used the reaction stops and BaF₂ beginsto grow epitaxialy. The BaF₂ growth begins as islands of about 4.0nanometer thick that merge. Once the islands merge, the rest of the filmgrows in a layer-by-layer mode. The growth of BaF₂ can be controlled soas to prevent the islands merger.

After the carbon nanotubes are deposited on BaF₂ covered silicon wafer,ends of carbon nanotubes touching the edge of BaF₂ islands can beidentified for comparison before and after thermal activation. A systemusing multiple growth chambers with a sample-preparation chamber and asurface-analysis chamber connected via vacuum interlocks can be used.

The specimen can be characterized with XPS before thermal activationusing monochromatized Al K-alpha radiation with a spot size of 1 mm anda spherical sector energy analyzer with an example total energyresolution was 0.51 eV. The specimen is transferred to a heater stationwithout breaking vacuum. To achieve thermal activation, the specimen isheated, for example, at 900° C. for two hours in a pressure of 10⁻⁹mbar. Since BaF₂ sublimates from Si at 800° C., at a temperature of 900°C. fluorine will be exposed to hot carbon nanotubes. After the specimenis cooled to room temperature, it is transferred to an analysis chamberfor XPS characterization before exposure to air. After the specimen isremoved from the MBE system, the specimen is placed in the AFM and sitesidentified for characterization before thermal activation arecharacterized for comparison.

A feature of the carbon nanotubes bundle (a loop with the shape of theinfinity symbol) facilities comparison, for example, an end of a bundleof carbon nanotubes touch the corner of a BaF₂ island grown on (001)silicon. The BaF₂ islands are substantially equilateral triangles withthe sides measuring of the order of two microns in length, and averagethickness of the islands of the order of 5.4 nanometers. AFM heightanalysis can be used to determine an average thickness of the islands. Asection of the carbon nanotubes bundle that lies on the BaF₂ islandbefore heating are not present after the heating. In addition, thecarbon-nanotube bundle is connected to the surface where there was anedge of the island.

Further, the carbon nanotube bundle is thinner after heating with aplurality of bumps on the surface of the bundle. These results areconsistent with a chemical reaction scenario where BaF₂ attacks thecarbon nanotube surface leaving a residue. In one implementation afterheating, fluorine is no longer present. However, different chemicalstates of barium, e.g., are present (metallic, Ba—Si, and Ba—C) onestate of Si (Ba—Si) and two states of carbon (Ba—C and graphite). Theresultant structure has a significant reduction in the carbon signalafter heat that indicates loss of carbon atoms during the reaction.There is a shift toward higher binding energy after the carbon nanotubesare heated due to the weaker c-c bond produced by the curvature of thenanotube. Following the heat treatment, there may be a second peakcorresponding to a carbide like bond. The changes in the spectrum afterheat indicate that during heating BaF₂ from the islands evaporate andreact with the carbon nanotubes.

Although some embodiments have been shown and described, it will beappreciated by those skilled in the art that changes may be made inthese embodiments.

1. A nanolaboratory, comprising: a substrate; a substance of fluoridesalt attached to the substrate; and a carbon nanotube positioned on thesubstance and at least partially merged with the substrate.
 2. Thenanolaboratory according to claim 1, wherein the substrate is a siliconcrystal or gallium arsenide (GaAs).
 3. The nanolaboratory according toclaim 1, wherein a first radical of the substance is selected from afamily of a periodic table having a spherically-shaped probabilitydistribution valence orbital.
 4. The nanolaboratory according to claim3, wherein the first radical is selected from Family II of the periodictable.
 5. The nanolaboratory according to claim 1, wherein the fluoridesalt is barium fluoride.